Synergistic effects of pilot-fuel characteristics and split injection strategies on combustion and emissions of the ammonia dual-fuel engine

High-reactivity pilot-fuel ignition is a promising approach for achieving efficient and clean ammonia combustion. The properties of pilot-fuel strongly influence combustion characteristics, while injection strategies accommodate distinct fuel characteristics by modulating mixture stratification. However, how pilot-fuel properties interact with split-injection parameters remains insufficiently understood. In this study, three pilot-fuels-diesel, biodiesel, and indirect coal-to-liquid (ICTL)-with distinct cetane numbers, oxygen contents, and volatilities were investigated to clarify their interactions with split-injection in an ammonia dual-fuel engine. Results show that split-injection improves ammonia combustion relative to single injection, but the gain depends on pilot-fuel properties and operating conditions. Diesel achieves the best trade-off between indicated thermal efficiency (ITE) and CO emissions. ICTL yields the greatest ignition delay (ID) reduction but the highest CO emissions. Biodiesel produces the lowest particulate matter (PM) but the highest particle number (PN). Further parametric optimization identifies a common optimal injection ratio of 20%–30% and optimal timings of −45 to −40 °CA ATDC for pre-injection and −1 to 1 °CA ATDC for post-injection, across all three fuels. Nevertheless, the dominant regulation mechanism varies with pilot-fuel properties. For high-volatility fuels, the post-injection ratio primarily governs ITE and PN; for high-cetane fuels, injection timing plays a dominant role in controlling the combustion phasing; and for oxygenated fuels, post-injection helps compensate for poor atomization while reducing PN. Pre-injection yields greater ITE gains at low ammonia energy fractions; post-injection becomes more advantageous at high fractions. These findings offer mechanistic support for developing fuel-specific split-injection strategies for ammonia dual-fuel engines.